You can find the five MX record mailserver domains, priority numbers, and recommended TTL on the Google Apps website.

Note: Don’t include a period at the end of the mailserver domain when you
enter it. The system will add that for you behind the scenes if it’s
required.

Click Add Record.

After adding the first MX record, repeat the process for the other four MX records.

Check and test

Now that the MX records have been stored within the DNS settings for
your domain, the changes should propagate after the time specified in
the TTL has passed. You can test your changes with a DNS checker.

Send an email through the Google Apps webmail interface to an email
address you can access on a different domain, then reply to it. If the
email to your domain gets back to Google Apps you’ll know the DNS
changes worked.